1. Field of the Invention
The invention relates to polymethacrylimide foams with improved thermomechanical properties.
2. Description of the Background
Polymethacrylimides having a high level of heat shape retention can be prepared by reaction of polymethyl methacrylate or its copolymers with primary amines. Representative publications of the large number of publications which disclose polymer-analog imidization include U.S. Pat. No. 4,246,374, EP 216 505 and EP 860 821. Further, JP 05222119 discloses that a polymer imidized with aryl amines have a high level of heat shape retention, while EP 561 230 and EP 577 002 disclose that this characteristic can be achieved by formulating the copolymer using special comonomers. All of these reactions, however, do not produce foams, but rather solid polymers, which must be foamed in a separate, second step in order to obtain a foam.
Polymethacrylimide foams have been known for a long time as disclosed, for example, in DE 27 26 260. They have been widely used, for example, as the core material for layered materials or foam laminates, because of their good pressure resistance and their low weight as disclosed in DE 28 22 885, DE 33 04 882 and U.S. Pat. No. 4,316,934.
Particularly in connection with the production of sandwich components with cover layers made of carbon fiber/bismaleinimide (CF/BMI) prepregs, there are high requirements with regard to mechanical stability at high temperatures. Typically, such components are first cured in an autoclave (several hours at up to 200.degree. C.), and then additionally tempered for several hours at approximately 240.degree. C. These extreme conditions can only be met by hard foams which demonstrate good heat shape retention and good creep behavior.
EP 532 023 A1 describes poly(meth)acrylimide foams having good heat shape retention and a uniform cell structure, which are obtained by heating polymers prepared from a) 5-50% tert.-butyl (meth)acrylate and 50-95% of a mixture of b) methacrylic acid and c) methacrylonitrile. In addition to the fact that when the polymer is heated, decomposition of the polymer occurs to the extent that isobutene is discharged therefrom, which is problematical for safety reasons, these foams particularly do not possess the high level of heat shape retention which is required for the production of laminate materials.
The required high level of heat shape retention is also not achieved by the products described in DE 18 17 156, which are obtained by foaming polymerizate sheets which consist of a) a polymerizate of at least 20 wt. % acrylic or methacrylic acid and, if necessary, one or more other unsaturated monomers, generally acrylonitrile or methacrylonitrile, and b) formamide or monoalkyl formamide. While it is mentioned in the specification that further improvements in properties can supposedly be achieved by the use of lower proportions of compounds with at least two reactive groups in the molecule, this is not documented with examples.
Another improvement in the material properties and particularly in the ability to produce imide foams is described in DE 27 26 260. By using 0.01-5 wt. % metal salts of acrylic and/or methacrylic acid, the task of being able to produce foams with a homogeneous cell structure and high density with non-reduced propellant amounts, even at high foaming temperatures, is accomplished. High foaming temperatures are absolutely necessary for good mechanical strength of the foam when it is later subjected to temperature stress in use, but if the propellant amount is unchanged, foaming is too strong and, therefore, the density is too low. Therefore, it has been possible to produce imide foams which are able to withstand thermal stress only by greatly reducing the amount of propellant, with a resulting loss of homogeneity of the cell structure.
The surprising effect of the metal salts is that it is possible to produce homogeneous imide foams with high density in their presence, with non-reduced propellant amounts, even at high foaming temperatures, is explained by reversible ionic cross-linking of the (meth)acrylic acid units with the metal ions. The salts obviously hinder foaming of the polymer, because of their bridging effect between the molecules, and thereby counteract the propellant.
However, no teaching is disclosed with regard to the heat shape retention of the imide foams as a function of the use of the metal salts of acrylic or methacrylic acid, and in the discussion of the examples and the tables, the foam density is only shown to be a function of the foaming conditions and the metal salt content. While it is true that in one case, an imide foam with heat shape retention at 246.degree. C. and a pressure strength of 5 N/mm is achieved by the use of chromium-III-dimethacrylate hydroxide, the use of such a toxic and ecologically unsafe metal salt is no longer in keeping with the times, and suitable alternatives are needed. Covalent, non-reversible cross-linking by means of multi-functional vinyl-unsaturated compounds is not mentioned in the patent.
DE 35 19 005 A1 discloses an improved polymethacrylimide foam, which is characterized by advantageous creep behavior at high temperatures and by a slight change in creep behavior when stored in humid air. This is achieved by heating a plastic, consisting of a mixed polymerizate of 47-53 wt. % methacrylonitrile, 53-47 wt. % methacrylic acid, and 0.01-4 wt. % of a metal salt of acrylic and/or methacrylic acid, as well as 0.5-4 wt. % of a propellant mixture of formamide or monomethyl formamide and a univalent aliphatic alcohol with 3-8 carbon atoms in the molecule, to temperatures of 200-240.degree. C. This significantly improves both the change in the creep behavior of the plastic in humid air and the water absorption of the plastic when stored in water. The use of additional comonomers is generally described as causing a worsening of the intended properties and is, therefore, not recommended.
Unpublished German Application No. 19606530.5 describes polymethacrylimide foams having improved flame protection. To obtain high values for the so-called lower limit for oxygen concentration, which is indicated in the form of LOI values (Lowest Oxygen Index), a high concentration of conventional flame protectants previously have had to be added, which necessarily causes a clear deterioration of the mechanical properties of the flame-protected foams. In accordance with the teaching of this application, the loss in properties usually caused by the high concentration of flame protectant can be circumvented by additionally adding an epoxy resin to the reaction mixture for production of the foamable polymerizate sheet, in addition to a relatively small amount of a conventional flame protectant. Because the concentration of additives is clearly reduced, in total, higher LOI values of at least 25 can be achieved while the material properties are not significantly worse. While it is true that the use of metal salt additives, metal salts of acrylic and methacrylic acid, as well as small amounts of cross-linking agents, is described, the values for heat shape retention, at less than 210.degree. C., which are achieved in accordance with this invention, are clearly too low for the requirements of the present application.